Our future is being threatened by exhaustion of fossil fuel resources, increasing cost pressure of energy due to reduced production and cost advancing pressure from oil-producing countries, and serious environmental pollution coming out from energy consumption.
In addition, conventional power generation system using fossil fuels, there are various power generation systems using various kinds of energy resources such as nuclear energy, tidal energy, water energy, wind energy, solar energy, bio energy, and so on.
However, nuclear energy even having economic feasibility has been restrictively developed only in some countries due to the Nuclear Nonproliferation Treaty and radioactive contamination, meanwhile water energy and tidal energy require proper site location satisfying system requirements, anticipated excessive investment and long-term construction period, while solar energy and wind energy require storage cell due to intermittent generation and higher cost. Accordingly, development of novel power generation system using clean energy is still needed.
Considering those systems consuming fossil fuels, future-oriented new power generation systems using clean energy resources must be competitive in construction costs and operation cost to the conventional electric power systems including land occupations, anticipative investments, construction periods, social costs from environmental pollution, and so on. In addition, the novel power generation system must have high annual operation rate and be free from expensive storage equipment or auxiliary power generation.
Meanwhile, since water having heavier mass has higher kinetic energy than air having lighter mass the technology converting kinetic energy of moving seawater, i.e., research of generating electricity from wave power is now in advance. In particular, energy resources from sea wave are out of count. However, frequency and power of waves in onshore and offshore have high fluctuations according to environment of the locations and seasons, whereas relatively lower wave height often occurs according to season's weather condition. Accordingly, if the disadvantage of practice caused by lower wave height and uneven wave period could be eliminated, uncountable wave power will be secured at no cost.
Describing in brief, even though the wave height is low, research and technology to increase conversion efficiency enabling practical use of wave energy and technology to reduce conversion loss by shortening conversion process is still needed.
Technologies of converting wave power into energy have been opened already as a movable body type (including a raft type), an oscillating water column type (an air turbine type), a tidal pressure type, a tidal potential type (a setup type), an overtopping wave energy conversion unit type, and so on.
In a state in which a distinct technology for real commercialization is not brought out yet, but several countries including the United Kingdom are focusing on research, development, and commercial operation of developed systems.
However, most countries in the world have not yet focused on investment or research on wave power generation, and only small air turbine generators have been introduced installed at marine buoys and at wave bather. Due to high fluctuation of sea wave (wave period, cycle and height) coming out from varying weather and seasons, technologies of wave power generation had been out of interest in research and investment.
It has been reported that, excluding a type in which installation of a large-scaled structure is essentially needed or energy conversion efficiency is too low to be economically inappropriate, but converting vertical movement of waves into mechanical energy using rafts is the best technology in efficiency among highly applicable technologies.
However, the method in which rafts connected each with hydraulic cylinders tied by hinges to convert wave movement into hydraulic pressure from axial movement in cylinder has disadvantages resulting from short motion stroke of the hydraulic cylinder arm and lower conversion efficiency from sharply decreasing cylinder stroke on the lower wave height.
That is, referring to FIG. 1, a radius of a quarter circle represents a maximum motion stroke of a cylinder, and it will be appreciated that while variation in momentum in a vertical direction is increased as the wave height is increased, the variation in momentum is sharply decreased when the wave height is small. That is, the cylinder motion stroke is abruptly reduced when the wave height is lowered, and thus, the energy conversion efficiency is sharply reduced. In other words, since energy is in proportion to the square of a velocity, the movement velocity of the cylinder is reduced, and thus, the wave power generator cannot be easily adapted to the waves with a low wave height. In FIG. 1, a vertical motion stroke ‘a’ of the raft corresponds to a cylinder motion stroke ‘a’, and a vertical motion stroke ‘b’ of the raft corresponds to a cylinder motion stroke ‘b’.
Eventually, various conventional wave power generators in stages of development and practical use have various difficulties such as inactive investment and research caused by fearing low efficiency of energy conversion and uncertainties of natural environment and energy outputs, and impracticability caused by following high investment and low return, and thus, supplements thereof are needed.
Meanwhile, as described above, while technology converting wave energy into kinetic energy by rafts raised and lowered according to wave level has large variation in output and instability against windstorms, still considering higher conversion efficiency and lower manufacturing cost, the technology using moving bodies may be most advantageous in economic feasibility and efficiency.
FIG. 2 shows a hinge-type hydraulic conversion apparatus among various conventional wave power generation systems in which a pair of rafts 1101 and 1102 are provided, and a hydraulic cylinder 1104 is connected to the rafts via hinges 1103.
In FIG. 1, a radius of a quarter circle represents a maximum motion stroke of the cylinder, and it will be appreciated that while variation in momentum in a vertical direction is increased when the wave height is increased, the variation in momentum is sharply decreased when the wave height is small. That is, the cylinder motion stroke is abruptly reduced when the wave height is lowered, and thus, the energy conversion efficiency is sharply reduced.
In other words, since energy is in proportion to the square of a velocity, the movement velocity of the cylinder arm is reduced, and thus, the wave power generator cannot be easily adapted to the waves at low wave height. In FIG. 2, the vertical motion stroke F/A of the rafts 1101 and 1102 corresponds to the cylinder motion stroke C/A, and the vertical motion stroke F/B of the rafts corresponds to the cylinder motion stroke C/B. In addition, reviewing the difference in vertical and horizontal displacement of the rafts 1101 and 1102, that is, comparing the horizontal displacement at higher wave with the horizontal displacement at lower wave height, it can be seen that there is a large difference between a stroke when the wave height is low and a stroke when the wave height is high.
FIG. 3 shows a Pelamis-type hydraulic conversion apparatus, among conventional wave power generation technologies, in which a pair of rafts 1201 and 1202 are connected by a hinge 1203 and hydraulic cylinders 1204 are connected to each other outside the hinge 1203.
Considering that a wavelength W/L is generally larger by several tens of times than a wave height W/H, the wavelength W/L is larger and the wave height W/H is much smaller. Here, a stroke of the hydraulic cylinder is represented as a numerical value calculated by multiplying the wave height W/H by the distance between the hydraulic cylinders/the wavelength W/L. For example, when the interval of the hydraulic cylinders 1204 of the of the rafts 1201 and 1202 is 2 m in the waves with the wave height W/H of 1.5 m and the wavelength W/L of 30 m, a maximum stroke of the cylinder is 2×1.5/30=0.1 (m). Since the maximum stroke of each of the cylinders is 10 cm, both sides of the cylinder strokes are merely 20 cm. In other words, even though the pressure is high, when the stroke is small and a flow rate is also small, driving efficiency of a hydraulic motor should be lowered. Ultimately, in coastal environments in which the wave height W/H is small, a simple system as shown in FIGS. 2 and 3 cannot be easily adapted.
Nevertheless, it is reported that the moving body type (a raft type, a pendulum type, etc.) among the various wave power generation systems has greater efficiency in power generation than the oscillating water column type (an air turbine type), the overtopping wave energy conversion unit type (a water turbine type), the tidal pressure type, a wave pump driving type, a buoy-generator direct connection type, and so on. This is referred to in marine energy engineering. Accordingly, when a technology in which structures of the rafts and levers are effectively improved, energy conversion steps are reduced, mechanical stability and conversion efficiency are increased and output variation can be controlled is developed, the wave energy may be likely to be one of the most competitive energy resource.